OPSONIN-INDEPENDENT LIGATION OF Fcy RECEPTORS

Recognition of bacteria by phagocytes is crucial to host defenses against infection. The elimination of bacteria generally occurs by specific receptormediated endocytosis and intracellular digestion . In the immune host, phagocytosis by Fc receptors and complement receptors is mediated by the presence of IgGand certain complement components on the bacterial surface. In the absence of these opsonins, other mechanisms, including carbohydrate-lectin interactions and adherence-promoting receptors, provide important alternative means for recognition and ligation (1-3). Two pathways of opsonin-independent phagocytosis use carbohydrate-mediated recognition mechanisms . First, lectin-like molecules on the surface of phagocytes can recognize specific carbohydrates on target particles. These receptors for carbohydrate include the ß-glucan receptor, which recognizes activators of the alternative complement pathway (4), and the mannosyl/fucosyl receptor, which ligates mannose on the surface of zymosan particles and on Leishmania donovani (5-9). A second mechanism of opsonin-independent recognition involves the adherence of lectin-like substances on the surface of bacteria to carbohydrates on the surface of phagocytes . For example, many Gram-negative bacteria, such as Escherichia coli, Klebsiella, and certain species of Salmonella and Shigella possess mannose-specific surface adhesions by which they bind to mannose residues on phagocytic cells . When this binding involves appropriate cell surface receptors, it may trigger internalization of the receptor-ligand complex in a manner similar to that of an immune opsonin-receptor complex with subsequent killing of the pathogen (10-13). The cell surface molecules with mannose residues available for binding to the mannose-specific adhesions on microbes have not yet been fully characterized. We have shown that the binding of Con A-treated erythrocytes (E-Con A) to freshly explanted human monocytes is dependent on the specific interaction of Con A with surface mannose on the phagocyte, and that Fcy receptors on human monocytes mediate the internalization ofthese E-Con A (14) . These observations led to the hypothesis that an Fc receptor, containing carbohydrate moieties with